wallet: add an optional mnemonic-entropy secret to the key stores

Additive plumbing for storing a BIP39 entropy alongside the HD seed, mirroring
how the seed itself is handled through both key store layers: a plaintext member
on CBasicKeyStore, an encrypted pair on CCryptoKeyStore, encryption during the
unencrypted-to-encrypted conversion in EncryptKeys with the plaintext cleared,
and decryption on unlock.

RawHDSeed and CKeyingMaterial are the same secure_allocator vector type, so the
entropy passes through EncryptSecret/DecryptSecret with no adaptation.

Nothing calls this yet; there is no behaviour change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/keystore.h

@@ -117,6 +117,12 @@ class CBasicKeyStore : public CKeyStore
 {
 protected:
     HDSeed hdSeed;
+    // BIP39 entropy for a mnemonic-recoverable wallet, kept BESIDE hdSeed, never
+    // instead of it. hdSeed holds the bytes actually fed to derivation (the
+    // expanded 64-byte BIP39 seed on new wallets); this record exists only so the
+    // seed phrase can be reprinted. Empty on legacy and hex-restored wallets,
+    // which is a normal state, not an error.
+    RawHDSeed mnemonicEntropy;
     KeyMap mapKeys;
     ScriptMap mapScripts;
     WatchOnlySet setWatchOnly;
@@ -129,6 +135,18 @@ public:
     bool SetHDSeed(const HDSeed& seed);
     bool HaveHDSeed() const;
     bool GetHDSeed(HDSeed& seedOut) const;
+    //! Mnemonic entropy: optional, present only on phrase-recoverable wallets.
+    //! Unlike the three seed accessors above -- which override pure virtuals on
+    //! CKeyStore (keystore.h:48-51) and therefore dispatch dynamically -- these
+    //! are plain non-virtual members: CKeyStore declares nothing for them and
+    //! nothing reaches the entropy through a base pointer. Every caller holds a
+    //! CWallet*, whose static type resolves to the CCryptoKeyStore overloads.
+    //! Do NOT add them to CKeyStore: that would force all four subclasses
+    //! (CBasicKeyStore, CCryptoKeyStore, CWallet, gtest's TestCCryptoKeyStore)
+    //! to implement them for zero call sites.
+    bool SetMnemonicEntropy(const RawHDSeed& entropy);
+    bool HaveMnemonicEntropy() const;
+    bool GetMnemonicEntropy(RawHDSeed& entropyOut) const;
 
     bool AddKeyPubKey(const CKey& key, const CPubKey &pubkey);
     bool HaveKey(const CKeyID &address) const